The invention relates to a process for recovering raw materials from wastes and residues, in particular for recovering heavy metals, in which a liquid or paste-like feed mixture and/or a feed mixture comprising comminuted or milled constituents is first produced and the recovery of the raw materials from the feed mixture is carried out by means of a chemical-thermal treatment and the waste gases are conveyed from the furnace through a multistage filter unit, where the first filter in each case is configured as a hot filter and the waste gases are subsequently cooled and flow through at least one second filter, are preheated and are then burnt at a high temperature. The invention further relates to an apparatus for carrying out the process.
Production processes, particularly in the production of a wide variety of products, result in formation of wastes or production-related residues, usually in the form of mixtures containing heavy metals such as chromium, zinc, nickel, copper, lead, etc., often in combination with organic substances, with the proportion of chromium generally predominating.
The separation of these mixtures to recover valuable raw materials is desirable, but is very difficult because of the wide differences in the chemical properties of these mixtures. For example, chromium(VI) oxide or chromium(III) oxide (Cr2O3) or zinc can be present in the liquid, paste-like or solid comminuted or milled wastes or residues.
Since the recovery of the heavy metals is very complicated and comparatively uneconomical, wastes containing heavy metals are, for example in a physicochemical plant, treated in such a way that the harmful materials are reduced, i.e. made inert, in other words made difficult to leach. The filter cake obtained in this way can then be stored permanently in a repository for hazardous waste.
For example, a process for recovering chromium known from Bayer AG is operated in a high-concentration region and only certain solutions which have been formed in Bayer""s own processes, which contain particular contaminants and which have been used as additives for processing chromium ores are used. Here, the heavy metals are brought from the liquid to the solid phase, i.e. the first step is a chemical treatment in which the highly concentrated chromium-containing wastes are precipitated to form sludges. After these sludges have been dried, the wastes can then be processed further in a metallurgical plant.
DR-A-3514471 discloses a process and an apparatus for separating arsenic from the hot waste gas formed in the metallurgical processing of arsenic-containing materials in melt processes. In this process, the waste gases, which contain arsenic-containing impurities or condensates and dusts containing valuable metals, are purified in a multistage filter unit. To achieve deposition of arsenic-free dusts of valuable metals, the waste gas is cooled immediately after the melt process and treated by addition of a reducing agent so that unstable arsenic-containing compounds are formed. As a result, the arsenic is kept in the gas phase in stable form and cannot deposit on the valuable-metal-containing dusts which are separated out in the hot gas filter.
The arsenic condensates are then separated off in a second, downstream cold filter.
However, such a process does not make it possible to recover heavy metals from waste and residues.
Furthermore, WO-A-91/05681 describes a process for recovering chromium from wastes by formation of water-soluble chromates. For this purpose, dried chromium-containing material is comminuted and heat-treated with addition of alkaline reactants and of oxidants in a closed chamber to which oxygen is supplied. The water-soluble chromates formed are subsequently rinsed out. However, this process is not suitable for producing dusts or flakes containing noble metals.
Finally, U.S. Pat. No. 3,968,756 describes a process for the incineration of chromium-containing sludge.
Otherwise, highly concentrated wastes are, in accordance with current strict environmental regulations, packed in containers or the like and preferably stored permanently in underground repositories.
In the case of slightly contaminated wastes which have a certain minimum calorific value, it is possible for them to be burnt in a waste incineration plant. However, the residues formed in the incineration, for example fly ash and slag, then have to be stored in repositories for hazardous waste because of the increased heavy metal content. In addition, the waste incineration plant has to be connected to a downstream afterburning plant in order to remove the pollutants, e.g. dioxin, formed in the first combustion of organic substances from the waste air. For this purpose, the afterburning plant has to be operated at a considerably higher temperature than the actual waste incineration plant. The disadvantage of this is that, as a result of the high temperature, the chromium(III) oxide produced in the first thermal treatment is converted back into readily soluble chromium(VI) oxide and can then even leave the plant in an uncontrolled fashion. A similar effect occurs in the case of zinc.
In exceptional cases, small amounts of the residues having a restricted composition can be used as additives in metallurgical plants after appropriate pretreatment.
These examples show that only an extremely small proportion of the valuable heavy metals is reused. The permanent storage of the major part of the heavy metals from residues and wastes in repositories for hazardous waste is the economically and ecologically least favourable variant.
It is an object of the invention to develop a process for recovering and separating raw materials from wastes and residues, which can be implemented using simple means operates very reliably and as far as possible forms no further wastes or residues and by means of which the residues and wastes which have already been stored in repositories can be reprocessed.
According to the invention, the separation of the raw materials is carried out in a process of the type mentioned at the outset by means of a chemical-thermal treatment in a furnace through which air flows, by first mixing, as a function of the composition, the liquid, paste-like and/or solid feed mixture with additives blowing it together with an oxidizing or reducing agent into the furnace. Subsequently, the feed mixture is subjected to the chemical-thermal treatment in the furnace by first mixing, as a function of their composition, the liquid, paste-like and/or solid feed mixture with additives, blowing the feed mixture together with an oxidizing or reducing agent into the furnace in which the furnace atmosphere flows at a predetermined flow velocity through the feed mixture at a predetermined temperature so as to form, as a function of the air composition and temperature, low-density flakes containing heavy metals which are conveyed by means of the air flow velocity established in the furnace through the filter unit.
Suitable additives are aluminium-, iron-, chlorine- or sulphur-containing materials, and also milled plastics or granulated plastics as reducing agents. For use in chromium-containing chamotte bricks, possible additives are aluminium oxide in a ceramic mix or iron oxide in the case of specific alloys.
The oxygen content, the rate of the chemical reaction and the density of the flakes formed are the most relevant process parameters and determine the necessary flow velocity in the furnace in the individual case. In addition, the flow velocity is dependent on the heavy metal to be recovered and also on whether a rotary tube furnace or a fluidized-bed furnace is used.
The thermal treatment is carried out in a reducing/oxidizing atmosphere at a temperature in the range from 350xc2x0 C. to above 700xc2x0 C. depending on the respective feed mixture. The furnace temperatures required in the individual case depend on whether heavy metals are to be recovered, on the composition of the feed mixture and on the end products which are to be obtained (mineralization temperature). Thus, in the case of chromium as chromium(III) oxide or oxidic mixtures, a furnace temperature of 500-900xc2x0 C. is required. In the case of zinc oxide, the most favourable temperature is 550-1250xc2x0 C. Furthermore, a reducing atmosphere is required for the recovery of chromium(III) oxide from chromium-containing residues and an oxidizing atmosphere is required for the recovery of zinc oxide from zinc-containing residues.
The temperature in the first filter is about 800xc2x0 C. The waste gas passing through the first filter can then be cooled to about 200xc2x0 C. before reaching the next filter.
The flue gas formed, which can contain CO2, SO2, Cl2, etc., is, after filtration, treated further in a customary flue gas purification plant to recover hydrochloric acid and sulphuric acid.
In chlorine-free systems, chromium(III) oxide is formed via the decomposition of the chromium-containing materials and the reduction/oxidation of the chromium.
In the case of chlorine-containing systems, chromium(III) oxide is formed via the decomposition of the chromium- and chlorine-containing materials, the formation of chromyl chloride (CrO2Cl2), the decomposition of chromyl chloride and the formation of chromium(III) oxide.
The recovery of chromium as chromium(III) oxide from wastes having a complicated composition is carried out by extraction with chlorine, with chromyl chloride being formed as intermediate.
The process can be used for the recovery of any heavy metals such as chromium, zinc, copper, lead, nickel, etc., with only the process parameters having to be adapted accordingly.
The process of the invention can most readily be implemented using a thermal reactor for the chemical/thermal treatment of the raw materials, where a multistage filter unit is connected immediately downstream of the reactor and an afterburning chamber is connected downstream of the filter unit in the flow direction of the waste gases or is arranged downstream of the dust separation, wherein the thermal reactor is a rotary tube furnace or fluidized-bed furnace whose furnace temperature is, depending on the mineralization temperature of the end products to be produced, in the range from 350xc2x0 C. to 1250xc2x0 C., the temperature of the first filter of the multistage filter plant is about 800xc2x0 C. and the second filter is operated at a temperature of about 200xc2x0 C.
In order to avoid undesired chemical reactions in the filter unit, the separation of dust or the flakes is carried out in the first filter at the temperature of 800xc2x0 C. and the flue gas or waste gas is cooled before reaching a further filter in order to prevent formation of chromium(VI) oxide again. As filter unit, it is possible to use all known filters, for example a cyclone or ceramic filter for high temperature (as first filter) or a textile filter for low temperatures (as second filter), with the waste gas being cooled to about 200xc2x0 C. before reaching the textile filter.
For the combustion or decomposition of organic compounds, very high temperatures have to be reached in the afterburning chamber so that only nonhazardous substances such as CO2, NO2 or SO2 leave the afterburning chamber. For this reason, it is advantageous to arrange a heating device between the filter unit and the afterburning chamber so that the necessary high temperature is achieved during the short residence time of the waste gases in the afterburning chamber.
The advantage of the process of the invention for recovering raw materials from wastes and residues is that the chemical-thermal treatment can be readily adapted to the different composition of the feedstocks by simply altering the process parameters and/or the composition of the additives. In addition, no further residues apart from the production-related wastes are formed. After the chemical-thermal treatment, only milling and sieving processes are necessary to prepare the recovered raw materials in the form of flakes having different particle sizes and densities for their further use, i.e. the recovered heavy metals are supplied to users in powder form.
Moreover, the process of the invention is very environmentally friendly since no environmentally damaging materials are formed at the end of the process or are released.
In the case of faulty batches, a repeated pass through the process can be provided. This becomes necessary, for example, in the event of an excessively high chromium(VI) content being found in the end product, i.e. in the flakes. In this case, another pass through the process can be carried out with addition of a reducing agent to ensure a product of acceptable quality.
The apparatus for carrying out the process of the invention is characterized in that a thermal reactor is provided for chemical-thermal treatment of the raw materials and in that a filter unit is connected immediately downstream of the reactor and an afterburning chamber is connected downstream of the filter unit in the flow direction of the waste gases or is arranged downstream of dust separation.
The thermal reactor is preferably configured as a rotary tube furnace or fluidized-bed furnace, so as to ensure a sufficient residence time of the wastes and residues mixed with the additives in the thermal reactor.
Furthermore, the filter unit has a multistage configuration with a cooling device connected downstream of the first filter. To avoid undesired chemical reactions, the temperature of the first filter is about 800xc2x0 C., i.e. the first filter is configured as a hot filter.
The second filter connected downstream of the first filter is configured as a textile filter and is operated at a temperature of about 200xc2x0 C.
Since the waste gases leaving the second filter have a very low temperature, it is advantageous for a heating device for preheating the waste gases to be arranged between the filter unit and the afterburning chamber. This ensures that the waste gases reach the necessary high temperature in the afterburning chamber.